1. Field of the Invention
The present invention relates to the dissemination of information about critical atmospheric conditions within the navigation message of regional and global navigation satellite systems (GNSS).
2. Description of Background and Other Information
One of the major sources of error for the measurement of GNSS ranging signals is the propagation delay caused by the atmosphere. While the ionospheric delay can be significantly reduced using two-frequency measurements, the delay caused by the troposphere is either completely ignored or statically modeled by the user. Due to a lag of information, actual atmospheric conditions are not considered for this modeling which results in a large contribution of the respective modeling error of the user ranging error in case of abnormal atmospheric conditions. From the perspective of accuracy and integrity, large margins are necessary in the system design to cover those very rare cases of mismodelling.
For the Galileo user receiver (user segment) a static set of parameters and a model is defined which is implemented in the receiver and used to model a major part of the tropospheric signal propagation delay. Parameters for a simple ionospheric model are disseminated in the navigation message of the system. These models are used by the user to correct the atmospheric propagation delay of the received signal. Due to (a) the static parameters (for troposphere) and (b) the simplicity of the model (for ionosphere) both strategies are limited to nominal atmospheric conditions only.
In other words, the prior art provides a solution to the problem of the atmospheric propagation delay of a received signal by correction of this delay. However, this correction of the prior art is limited to averaged nominal atmospheric conditions.
Correction of the atmospheric propagation delay of a received signal in critical atmospheric conditions are not known in the prior art. Within the meaning of the present invention, critical atmospheric conditions are meant to be situations for a specific user location for which the defined built-in correction models at the user receiver do not satisfy the user requirements or even lead to a non-availability of a specific service. Critical atmospheric conditions such as very low-pressure areas or short term ionospheric storms occur rarely but increase the modeling error of the user dramatically.
Since such critical atmospheric conditions will lead to a significant degradation of the quality of navigation, certain related services, or to integrity failures, it is necessary to detect such conditions and to alarm the user about such conditions.